The present invention relates to a track ball structure in which, when its ball is rotated, the rotation is decomposed into rotation of an X-direction component and rotation of a Y-direction component and signals representing the amounts and directions of the X- and Y-direction components are produced. These signals are entered into, for example, a personal computer to thereby move a cursor on its display screen.
FIGS. 1 and 2 show a conventional track ball structure. In a rectangular case 11 a ball 12 is rotatably supported and partly protrudes from the case 11 through an opening made in its top and an X-axis driven roller 13x and a Y-axis driven roller 13y, which are disposed at right angles to each other and have their peripheral surfaces frictionally coupled with the ball 12, are rotatably supported. The X-axis and Y-axis driven rollers 13x and 13y have X-axis and Y-axis rotary encoders 14x and 14y affixed to their shafts, respectively.
As the ball 12 is rotated, the X-axis and Y-axis driven rollers 13x and 13y are driven in accordance with the components of rotation in the X and Y directions, and signals corresponding to the amounts and directions of rotation of the X-axis and Y-axis driven rollers 13x and 13y are provided by the X-axis and Y-axis rotary encoders 14x and 14y, respectively. Where the rotary encoders 14x and 14y are of the optical type, light shielding plates 14x and 14y, each having slits of a predetermined pattern, are mounted on the shafts of the rollers 13x and 13y and a light emitting element and a light receiving element are disposed opposite one another across each light shielding plate. When the rotary encoders 14x and 14y are of the magnetic type, a plurality of magnets are arranged on each of the discs 14x and 14y circumferentially thereof and a magnetic sensor is disposed adjacent each disc in opposing relation to its circumference where the magnets are placed. When the encoders 14x and 14y are mechanical encoders, a printed circuit board having a predetermined conductive contact pattern is mounted on each of the rollers 13x and 13y and a contact piece is held in sliding contact with the patterned side of the printed circuit board.
In all the cases of such optical, magnetic and mechanical rotary encoders, rotary discs are affixed to the shafts of the rollers 13x and 13y.
Since in the prior art such a relatively large rotary disc is affixed directly to the shaft of each of the X-axis and Y-axis driven rollers 13x and 13y, the rotary disc (the above-mentioned light shielding plate or printed circuit board) which is used as each of the rotary encoders 14x and 14y is required to have a diameter, for example, equal to or more than 10 mm, for detecting the rotation with accuracy higher than a predetermined value. In order to house such relatively large rotary discs in the case 11, with their axes of rotation held at right angles to each other, and to keep them out of contact with the ball 12 in the case 11, it is necessary that the case 11 be sufficiently larger than the diameter of the ball 12 in both of the X and Y directions. Further, in the conventional track ball structure, the ball 12 is appreciably large in diameter, because it is required to sufficiently project upward from the top of the case 11 as well as to make contact with the driven rollers 13x and 13y which are rotary shafts of the rotary encoders 14x and 14y. In the prior art the track ball is embedded in an operating box provided separately of the keyboard, and hence need not be reduced in size. In the case where the track ball must be disposed on the keyboard, however, it is sometimes preferable to reduce its size in at least the X or Y direction so that it is of the same size as each key, for example.